1. Field of the Invention
This invention relates to a magneto-optical disk for large-capacity information storage or the like. This invention particularly relates to an improvement of the structure of a magneto-optical disk.
2. Description of the Prior Art
Among magneto-optical recording media, magneto-optical disks have attracted particular attention for enabling easy erasing and rerecording.
A magneto-optical disk is composed of a substrate, a magneto-optical recording layer for recording signals thereon and a protective layer constituted of a dielectric layer for protecting the magneto-optical recording layer, which layers are sequentially overlaid on the substrate. In general, signal recording and read-out are effected by irradiating recording light or reading light onto the magneto-optical recording layer through the substrate, which is constituted of a transparent plastic material.
The plastic material which constitutes the substrate absorbs ambient moisture and expands. Therefore, a stress imbalance arises in the plane of the substrate at the boundary between the substrate and the protective layer or the like having a small coefficient of moisture-absorption expansion. As a result, the magneto-optical recording layer or the protective layer cracks or separates from the substrate, and the quality of the performance of the recording medium deteriorates.
In order to eliminate the aforesaid problem, a novel magneto-optical disk has been disclosed in, for example, Japanese Unexamined Patent Publication Nos. 61(1986)-3336 and 62(1987)-20156. The disclosed magneto-optical disk is characterized by having a protective layer, which is constituted of an organic substance having high moisture barrier characteristics such as poly-para-xylylene or its derivatives or polyvinylidene chloride, on the outward side of the substrate, thereby preventing the substrate, from absorbing moisture.
However, to form a protective layer constituted of poly-para-xylylene or its derivative, a dimer of poly-para-xylylene is sublimated and decomposed into poly-para-xylylene monomers, and the monomers are then deposited onto the substrate. Therefore, from a practical point of view dense protective layers for blocking moisture cannot be mass-produced in an acceptable period of time because of the slow rate of deposition (approximately 1.mu./hour).
On the other hand, polyvinylidene chloride is classified into a solvent-soluble type and an aqueous dispersion type. When a protective layer is formed by use of the solvent-soluble type of polyvinylidene chloride, a problem arises in that substrates constituted of polycarbonate or acrylic materials (for example, PMMA) are corroded by the solvent used for dissolution of the polyvinylidene chloride. When a protective layer is formed by use of the aqueous dispersion type of polyvinylidene chloride, a problem arises in that the protective layer, which is formed by coating, whitens unless the substrate and the coating composition for the formation of the protective layer are heated. Also, a protective layer formed by coating readily cracks, depending on the concentration of the coating composition.
Also, forming a protective layer of a silicone resin, which is an organic substance having high moisture barrier characteristics, has been considered. However, when silicone resin is employed, a thermal cure process (at 120.degree. C. to 150.degree. C.) is required for condensation with dehydration during formation of the siloxane bonds, and productivity decreases because of the thermal cure process.
Further, it has been considered to constitute a moisture barrier layer of an ultraviolet-curing resin or an electron radiation curing resin.
When using an ultraviolet-curing resin, equipment for irradiating ultraviolet rays is required, and an additional process is necessary for curing resin.
When using an electron radiation curing resin, equipment for irradiating the electron beam is required, an additional process is necessary for curing resin, and productivity decreases.